1. Field of the Invention
The invention relates to a method for generating steam, in particular ultrapure steam, by reacting a stoichiometric mixture comprising a hydrogen-containing fuel and an oxidizing agent in a combustion chamber and injecting water into the hot reaction gases. In addition, the invention also relates to a steam generator for generating steam, in particular ultrapure steam.
2. Discussion of Background
In numerous technical application areas there is a need to provide steam with different temperature and pressure parameters.
For a range of applications in medical technology, food technology or experimental physics or chemistry, there is a need to provide steam in a very high level of purity within a very wide temperature and pressure range.
In addition to the conventional method of generating steam by boiling and evaporating water with subsequent superheating, it is also known to burn a stoichiometric mixture of hydrogen and oxygen in a combustion chamber and to inject water into the hot reaction gas so that it evaporates. In this way, very hot steam at very high pressures within a range of up to theoretically 3000 K and up to several hundred bar can be produced, unlike in conventional evaporation methods.
However, there are limits to this technology, as disclosed for example in DE 3512947 and DE 3936806, with regard to the purity of the steam which is generated. According to this method, the provision of steam of a very high level of purity requires virtually complete reaction of the starting materials hydrogen and oxygen which participate in the reaction. However, a problem in this respect is that, in view of the very high reaction temperatures, the additional water has to be injected directly into the combustion chamber, leading to local disruptions to the combustion operation, so that the reaction of the starting materials does not completely finish and the steam which is generated still contains a proportion of 20% to 30% of unreacted starting substances hydrogen and oxygen.
For many applications, in particular experimental physics, such a high level of unburnt substances cannot be tolerated.
Now, although it is obvious to take special process measures, for example relating to the injection of the water into the hot gases, to attenuate this quenching effect to some extent and thereby to achieve a higher degree of conversion in the oxidation reaction, nevertheless the steam which is generated in many cases, for example for the testing of combustion sequences under steam atmosphere, does not satisfy the extremely high purity levels required, meaning that many potential application areas of the use of ultrapure steam remain out of reach to this technology.
Accordingly, one object of the invention is to provide a novel method for generating steam of a very high purity which can be varied within a very wide pressure and temperature range.
Furthermore, the invention is based on the object of providing a steam generator which can be produced at low investment cost and which ensures complete conversion of the reaction mixture under all conditions.
According to the invention, the object is achieved by a method and a steam generator of the type described in the independent claims 1 and 11, respectively. Advantageous embodiments are given in the dependent claims.
The basic idea of the invention consists in configuring the exothermic reaction for providing the evaporation and superheating heat as a two-stage process in order to ensure complete conversion of the starting materials.
This is advantageously achieved with the aid of a method for generating steam, in particular ultrapure steam, by exothermic reaction of a fuel and an oxidizing agent, followed by cooling by the addition of water, by virtue of the fact that the hot steam-containing reaction mixture is subjected to catalytic afterburning downstream of the reaction and evaporation zone. In a preferred embodiment, the reaction mixture flows through a gas-permeable structure (referred to below as the through-flow body), which is equipped with a catalytically active surface, for example platinum.
To generate ultrapure steam, the preferred oxidizing agent is oxygen and the preferred fuel is hydrogen. Hydrogen peroxide is recommended as an alternative oxidizing agent. This applies in particular with regard to those applications of the ultrapure steam which is generated which are intended to reliably exclude even the slightest trace of oxygen.
For those applications of the steam which is generated in which a proportion of inert components is permissible in the steam, according to the invention the fuel hydrogen can be completely or partially replaced by gaseous or liquid hydrocarbons, in particular by natural gas, and the oxidizing agent oxygen can be completely or partially replaced by oxygen-enriched air.
To monitor the functioning and efficiency of the method, a lambda sensor for recording the oxygen content is connected downstream of the catalytic afterburning stage.
A steam generator for generating steam, in particular ultrapure steam, at least comprising a combustion and evaporation chamber having a reaction zone for the exothermic reaction of the fuel and an oxidizing agent and having an evaporation zone for evaporation and/or superheating of a quantity of water injected into the hot reaction gases is distinguished by the fact that a catalytic afterburning chamber is arranged downstream of the combustion and evaporation chamber.
In a preferred configuration, the catalytic afterburning chamber is designed as a cylindrical tube, the free cross section of flow of which is acted on, over a region of its axial length, by a through-flow body with a catalytically active surface.
In a particularly preferred embodiment, the through-flow body is based on a foamed metal material or a foamed ceramic material as substrate.
Alternatively, honeycomb-like or similar multicelled structures also achieve good results, provided that they offer a sufficient active surface area to the reaction gases flowing through.
The catalyst is in this case applied to the substrate as a coating or, in the case of a porous surface of the substrate, is incorporated therein.
In an expedient addition to the invention, the afterburning chamber comprises a double-casing tube which has cooling passages for indirect cooling by means of a fluid flowing through.
To prevent the formation of condensate at the chamber wall, it has proven expedient to use a gaseous cooling medium.
With the aid of the invention, it is now possible to generate steam of a very high level of purity, i.e. with a purity of over 99.9%, with relatively little technical outlay.
The ability to generate such a pure steam mixture, in conjunction with the high flexibility of the steam generator with regard to the process parameters throughput, temperature and pressure open up new potential application areas in research and industry, for example the testing of combustion processes under a steam atmosphere, the treatment of special waste or technology related to emission-free energy conversion, to the technology of superheated high-purity steam.
On account of its modular structure, the steam generator according to the invention can be matched to the requirements of different applications with little outlay on apparatus. It requires little maintenance, is environmentally friendly and is distinguished by low investment costs and low operating costs. It can be produced both on a large industrial scale and for applications on a laboratory scale.
In addition to the simple structure, the high flexibility with regard to the process and capacity parameters throughput, pressure and temperature and the availability of the installation should also be emphasized.
The steam generator is suitable for continuous operation, on account of its short response times, but in particular is also suitable for intermittent operation, since it reaches a steady operating state within a very short time of ignition. The response times to changes in the process parameters are extremely short.
A further advantage resides in the possibility of using a commercially available lambda sensor for the gas analysis.